Adhesion promoting compounds are used in commercial ink formulations in order to enhance the adhesion between the ink and the substrate onto which the ink is printed. Failure of adhesion leads to difficulties in the printing process or inadvertent removal of the print. Printing of polymeric food packaging is widespread and it is particularly important in such applications that the printed ink remains on the packaging and that parts of the ink composition do not contaminate the product.
Adhesion promoters based upon titanium compounds have been known and used commercially for many years. Whilst providing good adhesion between the ink and substrate, simple titanium compounds such as titanium acetylacetonates have tended to impart undesirable yellow colour and odour to the ink. GB-A-2161811 describes an adhesion promoter which is an organotitanate comprising the reaction product of a titanium orthoester and at least a monoalkyl phosphate (for example a mixture of a monoalkyl phosphate and a dialkyl phosphate) in which the alkyl group contains up to 6 carbon atoms and in which the total mole ratio P:Ti in the product is less than 2. Such compositions provide efficient adhesion promotion with much less yellowing and odour than the aforementioned titanium compounds and have been successful commercially. WO2005/049662 and WO2005/097919 each describe alternative adhesion promoters made by reacting together a titanium compound, a phosphorus compound and a resin.